Robots can be used in a multitude of tasks, but generally they require end effectors which are designed for specific tasks. Normally, the design and fabrication of specialized end effectors has been the responsibility of the robot user. However, in order to increase the versatility cf "off-the-shelf" grippers, more universal anthropomorphic designs are being developed.
One advanced design is a parallel jaw gripper which has as its main advantages simplicity in mechanical and electrical design and a desirable grip force-to-weight ratio. For example, force and position control can be obtained with a single actuator, one position sensor and one force sensor, and one microcomputer. Grip force-to-weight ratios on the order of 50:1 are easily obtainable as a result of the point of load application having no mechanical advantage over the linkage. Nevertheless, such grippers have several disadvantages, such as the inability of the gripper to reorient a part after it is grasped and the necessity of presenting an object to the gripper in a predetermined, known orientation. An additional disadvantage is the need for different types of hands for different parts and the requirement that the entire robot be disabled while lengthy adjustments in gripper hands are done.
Unfortunately, as with the robots themselves, there is usually no "best type" of design for a grasping system. It is important in any such design that the characteristics and movements of the overall system be considered. For example, in a precision flexible manufacturing work cell, finished parts having tolerances on the order of one-thousandth of an inch are usually stored in an orderly fashion to prevent the formation of burrs instead of merely being piled in a bin so human-like grasping abilities are not required. Other problems with respect to robots include payloads being quite limited in order to maximize the grip force-to-weight ratio and thus maximize a robot's potential to do useful work. When operating in a precision flexible manufacturing area, robot repeatability and accuracy must be high and reliable, but usually are not when compared to the precision parts ard fixtures with which they operate. Consequently, methods are necessary to assist the robot durirg parts insertion processes and parts manipulaticn processes. On the other hand, those manipulatcrs which have the flexibility and accuracy are usually extremely complex in both their mechanical and electrical systems, and consequently usually have a low grip force-to-weight ratio. As an example, a gripper currently being developed uses 38 pneumatic actuators controlled by 6 microprocessors that are coordinated by a minicomputer.
A manipulating device that utilizes a double threaded ball screw is disclosed in the U.S. Pat. No. 3,261,479 Baker et al. Such a device utilizes a stationary threaded screw which when turned, causes two bosses 19 and 20 to move together or apart so that grasping fingers 21 can engage an object 22.
A two finger manipulator for a robot is disclosed in the Inagaki et al U.S. Pat. No. 4,336,926. This patent discloses in column 1, lines 14-24 that it is old to use a motor driven screw rod with oppositely threaded portions to drive the robot fingers. Such a rod is also disclosed in a photograph of a precursor to the present invention on the cover of the January, 1984 edition of American Machinist.
Nevertheless, all of these prior art devices do not provide a highly versatile, mechanically and electrically simplistic robot end manipulator with the accuracies required in the machine tool manufacturing industry. Thus, there is need for an end effector or grasping system for use by robots which are operated in a machine tool environment. Such a gripper must not only have a high grip force-to-weight ratio, but should be thin, symmetrical, and manipulatable in a plurality of axes.